32 ESSENTIALS OF BIOLOGY 



ing, as inclusions, the highly characteristic chromatin bodies. 

 The above are the general characters of an undijferentiated 

 cell. 



gb. The Protist Body. Such may also be the general char- 

 acters of the unicellular organism, or Protist. But such general 

 characters are modified in detail so that there are multitudinous 

 forms of Protists, defined by variations in the internal and external 

 skeletal parts ; by differences in the size, form, etc., of the nuclei ; 

 by differences in the numbers and natures of the cell inclusions, 

 etc. These various forms of the Protist body can be arranged 

 into specific categories just as in the cases of the multicellular 

 organisms. The Bacteria are to be regarded as Protists, but 

 here morphological categories cannot be made — partly because 

 of the very minute sizes of the organisms, and perhaps because 

 the specific categories may be based on the physiologies of the 

 organisms. In practice categories of Bacteria are recognized by 

 the activities of the organisms in acting chemically on the nutrient 

 substances in their environments. 



The Ultra-microscopic organisms. The existence of these is 

 inferred. They are too small to be seen in the sense that an 

 ordinary Diatom, say, is seen. Their presence can be detected 

 microscopically but not their forms. There is a limit to the size 

 of particles that can be seen, even in the perfect microscope, and 

 this limit depends on the relative dimensions of light- waves and 

 those of the particles envisaged. The conception of definite 

 morphology therefore fails in these cases. 



10. THE MULTICELLULAR ORGANISM 



The bodies of the multicellular plants and animals are complexes 

 of cells ; the cells are differentiated into kinds and the units 

 of each kind are tissue-elements. Thus there are bone-cells, 

 muscle-cells, nerve-cells, gland-cells, connective-tissue-cells, 

 etc. Tissue-elements are compacted together into tissues and 

 tissues have tectonic arrangements as organs. Thus organs are — 

 skeleton shell, muscle-systems, sense-organs, etc. (see further in 

 Section 12). 



lofl. Symmetry of Parts. The higher multicellular organism 

 has parts which are more or less repetitional in structure, each 

 part containing all or many of the organs that are also in other 



